The present invention relates generally to medical devices and more particularly to delivery systems for medical devices.
Intraluminal medical devices are used by physicians to treat numerous conditions using minimally invasive procedures. Examples of intraluminal medical devices include stents, stent-grafts, filters, valves, etc. One type of intraluminal medical device that has become especially common is self-expanding stents. Typically, self-expanding medical devices, including stents, are made from an elastic structure that may be compressed into a low profile state that can be passed through vessels in a patient with minimal trauma. Once at the desired treatment site, the self-expanding medical device is released and self-expands like a spring until it contacts a tissue wall which prevents further expansion. Common materials that are used in self-expanding medical devices include nitinol and stainless steel, although other materials are also possible.
Self-expanding stents are used to treat various organs, such as the vascular system, colon, biliary tract, urinary tract, esophagus, trachea and the like. For example, stents are commonly used to treat blockages, occlusions, narrowing ailments and other similar problems that restrict flow through a passageway. One area where stents are commonly used for treatment involves implanting an endovascular stent into the vascular system in order to improve or maintain blood flow through narrowed arteries. However, stents are also used in other treatments as well, such as the treatment of aneurysms. Stents have been shown to be useful in treating various vessels throughout the vascular system, including both coronary vessels and peripheral vessels (e.g., carotid, brachial, renal, iliac and femoral). In addition, stents have been used in other body vessels as well, such as the digestive tract.
One type of delivery system for intraluminal medical devices includes an inner catheter and an outer sheath attached to a handle arrangement. One portion of the handle is typically connected to the inner catheter and another portion of the handle is typically connected to the outer sheath. The inner catheter extends coaxially through the outer sheath, and the two portions of the handle are arranged to longitudinally pull the outer sheath relative to the inner catheter. Thus, when the distal end of the delivery system is positioned within the patient's body at the intended treatment site, the physician actuates the handle outside the patient's body by moving the two portions relative to each other so that the outer sheath is withdrawn over the medical device and inner catheter. In the case of self-expanding medical devices, like stents, the outer sheath also serves to radially restrain the device in the compressed state until the outer sheath is withdrawn. As the outer sheath is withdrawn, the medical device is released in the body at the treatment site, and in the case of a self-expanding stent, the stent expands outward away from the inner catheter and presses against the vessel wall. Although the outer sheath is usually withdrawn by pulling the outer sheath proximally relative to the inner catheter, it may also be possible to withdraw the outer sheath by pushing the inner catheter distally relative to the outer sheath. After the medical device has been fully released from the delivery system, the handle may then be pulled by the physician to withdraw the inner catheter and outer sheath from the patient's body, while leaving the medical device implanted in the body.
Precise placement of intraluminal medical devices is a concern in most medical procedures. One problem that can contribute to imprecise placement of intraluminal medical devices is deflection of the delivery system during deployment. This can be a particular problem in the deployment of self-expanding medical devices, like stents, because the medical device presses outward against the inner surface of the outer sheath prior to deployment. When the outer sheath is withdrawn, the outward pressure exerted by the medical device creates friction between the medical device and the outer sheath. Since the medical device is typically prevented from moving proximally with the outer sheath by a stop attached to the inner catheter, the frictional force between the medical device and the outer sheath causes the outer sheath to be in tension and the inner catheter to be in compression. This can cause the inner catheter to contract in length due to the compressive force. In addition, the inner catheter can buckle, or snake, within the outer sheath. Both of these responses can cause the distal end of the inner catheter, and thus the medical device itself, to move proximally from the intended treatment site. Although the contraction and buckling may decrease somewhat as the outer sheath begins to withdraw from the medical device due to the release of some of the frictional force, the distal end of the inner catheter may not completely return to the intended treatment site when the medical device is initially released and implants within the patient's body. Moreover, the stent and/or inner catheter can build up sufficient spring force due to the contraction of the inner catheter and the stent to cause the stent to jump distally once the static friction is released. With medical devices that cause high frictional loads against the outer sheath, like drug coated stents, covered stents and particularly long stents, the initial deflection of the delivery system and subsequent distal movement due to the release of friction can make it difficult for a physician to predict the exact location where the medical device will be released in a patient's body.
Accordingly, the inventor believes it would be desirable to provide an improved delivery system for intraluminal medical devices.